1. Field of the Invention
The present invention relates to a high voltage power supply circuit for alternately changing between a positive and negative output.
2. Description of the Related Art
A conventional high voltage power supply circuit for changing between a positive or negative output employs both transistors for both positive and negative outputs and the positive and negative outputs are achieved by either transistor being selectively turned on or off.
The conventional high voltage power supply circuit has incorporated thereinto two transformers, one each for positive and negative outputs. There are disposed switching transistors at the respective primary windings of each transformer which are selectively turned on and off.
There are also disposed rectifier circuits consisting of a diode and a capacitor at the respective secondary windings of each transformer. Each rectifier circuit, in which the diode is connected such that the polarity is the opposite of the other, provides a positive or a negative output through an output terminal in response to turning on or off of the switching transistors.
This kind of high voltage power supply circuit employs a load resistor in a circuit providing a negative output to avoid the influence that circuit would have when a connected circuit providing a positive output generates a positive output, and on the contrary, a load resistor in the circuit providing a positive output to avoid the influence from the connected circuit providing a positive output when the circuit for a negative output generates a negative output. Accordingly, since output current flows through the load resistor, the output voltage will decrease when the output current increases so that the output load characteristic (regulation) of the high voltage power supply circuit is unstable.
To solve the above problem, there are provided, in the conventional high voltage power supply circuit, resistors between the rectifier circuit and the output terminal which divide the output voltage and the divided voltage is fed back to a control circuit which turns the switching transistors on or off. Accordingly, the output voltage should be regulated by controlling pulse widths provided to the transistors, which makes the high voltage power supply circuit complicated and large, thereby increasing the cost.
There also has been proposed a high voltage power supply circuit which avoids the inconsistency of the output load characteristic and eliminates the load resistors on the positive and negative outputs. The proposed high voltage power supply circuit is constructed such that the rectifier circuits providing the circuits for positive and negative outputs are connected through switches to the output terminal to eliminate the mutual influence of each circuit when a signal is provided at the output.
According to the above technology, no output current flows through the load resistor so that the output load characteristic is stable.
However, the use of a relay for switching between the positive and negative outputs is required, so that the high voltage power supply circuit becomes large and complicated, which increases the cost.
Further, since each of the foregoing technologies requires a plurality of the transformers to have a winding direction corresponding to the output polarity and also requires transformers for a high voltage, the high voltage power supply circuit will become complicated and large, thereby increasing the cost.
There is also disclosed, for example, in a Japanese Laid-open Patent Publication 63-64571 a high voltage generation device which does not utilize a transformer and does not employ a load resistor. However, the above device is principally a circuit for obtaining an output voltage having more than n-time voltages by using a voltage n-multiplier rectifier circuit, so it cannot perform the operation for changing over from positive to negative or vice versa. Accordingly, the above technology is completely different from the present invention in its circuit function and principle so that it cannot be utilized as a high voltage power supply circuit for an electro photographic printer or the like which requires a switching function.